Most modern cloud infrastructures have moved to standards-based identity providers so that identity can be easily shared between internal and external IT environments. The standards deployed by the various vendors include:
- OpenID: http://openid.net.
- OAuth: https://oauth.net/.
- SAML: Typically used in the enterprise environment to federate Microsoft Active Directory with cloud services. See https://en.wikipedia.org/wiki/Security_Assertion_Markup_Language for more information.
From a selection criteria perspective, if you are working with a cloud vendor and they do not support these standards, you should move to another vendor.